T lymphocytes play an important role in the immunopathogenesis of AIDS. Elucidating the molecular interactions of T lymphocytes and the AIDS virus is, therefore, of central importance for understanding the immune sequelae of an AIDS virus infection. An AIDS virus-encoded superantigen may play a role in facilitating viral replication, spread, and persistence and, thereafter, contribute to the viral pathogenicity. The simian immunodeficiency virus (SIV)-infected macaque monkey has proven to be an important model for the study of the pathogenesis and treatment of AIDS. A variant of SIV termed SIVsmmPBj 14 has been demonstrated to stimulate T lymphocyte proliferation in vitro and induce an acutely lethal disease in macaques characterized, in part, by lymphadenopathy and splenomegaly. These unique properties of this isolate raise the possibility that this virus may indeed encode a superantigen which may play a role in the acute, lethal disease it induces. To explore this issue, we will study at a molecular level the T lymphocyte response to SIVsmmPBj in macaques. In these studies we will assess: I. The SIVsmmPBj superantigen-coding gene. II. The role of the SIVsmmPBj superantigen in viral pathogenicity.